What you did was just cover up the rust, it will continue to spread under the under coating. You need POR 30 paint, it converts the rust to a paint. I've been slowly doing sections as needed. It's expensive, but it's final. It dries shiny but unless you spray a UV protectant on it will fade to flat black which matches the factory undercoating nearly perfect. Wear gloves cause nothing takes it off your hand, it will be on for your hands for days otherwise.
http://www.por15.com/POR-15/productinfo/1GB/